This invention relates to the synthesis of RF radiation pattern to create a phased array using a plurality of antennas and more particularly to a system and method for generating such RF patterns from a plurality of antennas using a number of linear power amplifiers (LPAs) less than the number of antennas and even more particularly to such systems and methods for using the same number of LPAs as are currently used in non-phased array antenna systems.
It is often desirable to provide a signal simultaneously in multiple beams of a multibeam antenna system. For example, a cellular communication system may provide communications between a base transceiver station (BTS), having an antenna system associated therewith, and a plurality of mobile units operating within a predefined area, or xe2x80x9ccell,xe2x80x9d defined by the antenna system""s radiation pattern. Often such cells, although providing communications in a full 360xc2x0 about the BTS, are broken down into three 120xc2x0 sectors in order to provide more capacity and less interference over that of an omni cell 360xc2x0 system. Additionally, such a sectorized cell achieves extended range as compared to an omni cell 360xc2x0 system due to the greater signal gain at the sector antennas resulting from their more focused coverage.
Further advantage may be realized by providing multiple narrow beams at the BTS rather than the three 120xc2x0 sectors. For example, twelve 30xc2x0 narrow antenna beams may be utilized to provide the same 360xc2x0 communication coverage within the cell as the 360xc2x0 omni cell configuration and its 120xc2x0 sectorized cell replacement. Such a multiple narrow beam arrangement is desirable because, as with the 120xc2x0 sector system described above, the multiple beams provide a greater signal gain resulting from their greater focused coverage. A further advantage of the multiple narrow beams is the flexibility offered in synthesizing any desired sector size by combining/phasing such beams. Combining adjacent narrow beams provides a wider composite beam, with a beam width roughly equal to the sum of the individual beams widths. Accordingly, synthesized sectors may be formed of any size by simulcasting a signal on selected ones of the narrow beams. The sector could be as narrow as a single beam or as wide as desired by using multiple beams.
The multiple antenna beams of a communication system may be generated through use of a planar or cylindrical array of antenna elements, for example, where a signal is provided to the individual antenna elements having a predetermined phase relationship (i.e., a phased array). This phase relationship causes the signal simulcast from the various antenna elements of the array to destructively and beneficially combine to form the desired radiation pattern. There are a number of methods of beam forming using matrix type beam forming networks, such as Butler matrixes.
Controlling interference experienced in wireless communication, such as may be caused by multiple users of a particular service and/or various radiating structures of a service or different services providing communication coverage within the same or different geographical areas, is a concern. Moreover, as the use of wireless communications increases, such as through the deployment of new services and/or the increased utilization of existing services, the need for interference reduction schemes becomes more pronounced.
For example, in code division multiple access (CDMA) networks a number of communication signals, each associated with a different user or communication unit, operate over the same frequency band simultaneously. Each communication unit is assigned a distinct, pseudo-random, chip code which identifies signals associated with the communication unit. The communication units use this chip code to pseudo-randomly spread their transmitted signal over the allotted frequency band. Accordingly, signals may be communicated from each such unit over the same frequency band and a receiver may despread a desired signal associated with a particular communication unit.
However, despreading of the desired communication unit""s signal results in the receiver not only receiving the energy of this desired signal, but also a portion of the energies of other communication units operating over the same frequency band. Accordingly, CDMA networks are interference limited, i.e., the number of communication units using the same frequency band, while maintaining an acceptable signal quality, is determined by the total energy level within the frequency band at the receiver. Therefore, it is desirable to limit reception of unnecessary energy at any of the network""s communication devices.
In the past, interference reduction in some wireless communication systems, such as the aforementioned CDMA cellular systems, has been accomplished to an extent through physically adjusting the antenna array to limit radiation of signals to within a predefined area. Accordingly, areas of influence of neighboring communication arrays may be defined which are appreciably smaller than the array is capable of communicating in. As such, radiation and reception of signals is restricted to substantially only the area of a predefined, substantially non-overlapping, cell.
Changes in the environment surrounding a communication array or changes at a neighboring communication array may require adjustment of the radiation pattern of a particular communication array. Specifically, seasonal changes around a base transceiver station (BTS) site can cause changes in propagation losses of the signal radiated from a BTS. For example, during fall and winter deciduous foliage loss can cause a decrease in signal path loss. This can result in unintentional interference into neighboring BTS operating areas or cells as the radiation pattern of the affected BTS will effectively enlarge due to the reduced propagation losses.
Likewise, an anomaly affecting a neighboring BTS may cause an increase in signal path loss, or complete interruption in the signal, therefore necessitating the expansion of the radiation patterns associated with various neighboring BTS""s in order to provide coverage in the affected areas.
One solution to the problem of creating a phased array has been to use twelve antennas arranged into three panels with each panel having four antennas thereon. A typical system of this type is shown in FIGS. 6 and 7 where it will be noted that there are at least twelve LPAs utilized, one for each antenna column. Also, it should be noted that any reference herein to an antenna or an antenna array includes an antenna column made up of a plurality of elements. Control of such an antenna column is detailed in the above-mentioned copending application entitled SYSTEM AND METHOD FOR PER BEAM ELEVATION SCANNING.
In systems which existed prior to the system shown in FIGS. 6 and 7, particularly in CDMA systems, there is typically only one LPA per panel (sector) which provides RF signals to a single antenna, or to a set of antennas having a relatively fixed radiation pattern. In such systems there is no ability to control, or snythesize, the radiation pattern to maximize utilization. For such synthesis to occur and thus control the radiation pattern, it is necessary to control the power and the relative phase of the RF signals which arrive at the antenna.
Thus, in addition to the LPAs being costly, if twelve LPAs were to be used (one for each antenna), their use would require the removal, or at least the rewiring of, the existing LPAs plus the addition of at least nine additional LPAs. This is costly and inefficient.
In addition, since LPA themselves are costly items (the cost partially dependent upon the amount of power being handled) and because the amount of power delivered to the antenna is critical to the proper operation of the system, it is critically important that power not be lost (or the loss minimized) after the LPA stage.
A further difficulty arises when it is desired to change the orientation of the radiation pattern so that antennas positioned on different panels create the sector. Thus, assume that the four antennas of each panel work together in a phase relationship to create three sectors each covering, for example, 120xc2x0. Now let us assume that it is desired to use two antennas from one panel combined in a phase relationship with two antennas from an adjacent panel to change the sector physical dimensions. In order to achieve this new relationship the synthesis of the signals must be changed to reflect the new radiation pattern.
Accordingly, a need exists for a multiple phase array of antennas to be controlled using a single LPA per panel.
Accordingly, a further need exists in the art for an antenna system in which RF signals are processed after the power amplification stage to provide synthesized signals while not creating a significant power loss.
A further need exists in the art for such a system in which the synthesis of radiated sectors can be created using antennas from different panels.
A still further need exists in the art where each panel of a multipanel antenna system has a single LPA feeding RF power to the panel.
These and other objects, features and technical advantages are achieved by a system and method where only a single LPA per panel is utilized to control RF signals to that panel. The circuitry between the LPA and the antenna panels serves to first double (split) and then double (split) again the output of the LPA so that there are ultimately four distinct signals, each having its own power and phase relationship, for presentation to the four antennas of the antenna panel.
The power levels are adjusted by the use of a two by two (two inputs and two outputs) hybrid with a phase adjustment in at least one of its inputs. The phase adjustment serves to control the relative power output between the two outputs. These two outputs are each provided to splitters for creating two signal pairs. Each signal pair is again fed through a two by two hybrid circuit and the relative phase of one signal of each pair is adjusted to yield the desired output power levels. Thus, there is created four signals (each for presentation to a different antenna) each having selective power, all coming from a single LPA. Phase control is applied to each of the four signals just prior to the antenna so as to control the physical dimensions of the radiation pattern.
Prior to the RF signal arriving at the single LPA associated with a particular panel there is provided, if desired, circuitry which allows the RF from any of the three inputs to be directed to any of the other panels, thereby allowing for cross panel control, as well as reversal, of the sectors.
Accordingly, it is one technical advantage of the invention that a single LPA controls multiple antennas of a sector, each antenna having a controllable power level and phase relationship with respect to the other antennas in that sector.
It is a further technical advantage of the invention that the power levels between the antennas of each sector is adjusted relative to each other so as to minimize power waste.
It is a still further technical advantage of the invention that the single LPA of each sector can be directed to control antennas in different sectors, thereby allowing greater flexibility in sector boundary management.
The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims.